1. Field of the Invention
The invention concerns a measuring turbine with a turbine rotor arranged in a flow channel of the measuring housing. The turbine rotor is positioned such that its axis is in the direction of the flow. The turbine rotor is also equipped with an inductive pulse pickup.
2. Background of the Prior Art
Flow meters of this type are known per se and belong to the group of the intermediate volume counters operating on the principle of the so-called Woltmann impeller wheel counter and being particularly suitable for the accurate measurement of the instantaneous volume flow of fluids. A turbine rotor with a low mass centeringly located in a tubular body is exposed to flow in the axial direction with the medium to be measured impacting the turbine disk in the form of a quasi laminar flow after having been quieted by means of flow rectifiers. The rpm of the turbine disk is proportional to the mean flow velocity and thus corresponds over a broad range to the volume passing through.
The rpm of the turbine disk is preferably taken off with a low reaction force through the non-magnetic tubular body by means of an inductive transducer. The number of pulses per unit time is proportional to the instantaneous volume flow, while even in the case of the minimum volume flow, the rpm of the turbine disk is not affected by the inductive pulse takeoff. However, hydraulic losses are dependent on viscosity and are functions of the Reynolds number, respectively.
By selecting high quality, low friction bearing materials and by reducing the impeller mass, efforts have been made to keep the braking torque as low as possible in order to preserve a maximum effect, i.e., a broad measuring range of the counter or expansion of the measuring range. This affects the characteristic of the error curve. Such measures, however, largely depend on the angle of the blades to the axis of the turbine disk and on the number of blades, whereby the circumferential velocity of the impeller wheel is also affected.
Attempts to make the known turbine flow meters into an accurately operating measuring instrument have led over the course of time to improvements concerning the configuration of the impellers.
There are known configurations with two different slopes, for example, wherein a steeper slope is used in the forward part of the disk than in the rear part. This results in the fact that in the case of small flow volumes the impact is more intensive so that even with flow volumes, rpm ranges are attained whereby the lower limit of the measuring range may be lowered. In order, however, to avoid reaching excessively high rpm ranges leading to increased wear, the rear part of the impeller disk is given with a less steep slope, thereby providing for the necessary equalization of velocity.
The reduction of the rpm of the impeller disk in the range of relatively low flows affects the characteristic of the error curve so that the latter may be maintained flatter, thereby making it possible to either reduce the error limit or to expand the measuring range, depending on whether the zero limit is to be shifted or its position maintained without change.
During the measuring process, the housing wall surrounding the turbine disk affects the rotating motion of the turbine. The fluid to be measured is at rest immediately adjacent to the wall, while on the circumference of the turbine disk high rpms of the fluid are attained. There is also, therefore, a relative movement of the flow medium in the circumferential direction of the turbine disk. If the circumferential velocity component of this flow is affected by suitable means, the existing rpm of the turbine disk is also altered.
It is known from the mathematical fundamentals indicating the qualitative relationship of turbine disk counters, that the slip of the turbine rotor decreases with the square of the flow. This type of slip occurs, for example, during the rotation of cylinders in housings with a narrow gap, but also during the rotation of blades in the vicinity of housing walls. It is known to utilize both processes in the construction of so-called viscosity compensated turbine disk counters. The flow in this instance is within the so-called laminar turbulent range with a slip effect of 1/Re, wherein Re is the Reynolds number.
Other effects are, however, superposed on the effect declining with the square of the volume flow of this slip, so that in actual practice there is a negative slope of the error curve (see the broken curve in FIG. 3 following the maximum).